Conventionally, a spark plug has been used to ignite an air-fuel mixture through spark discharge (may be referred to merely as “discharge”) for operation of an engine, such as an automotive internal combustion engine. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, development of a plasma jet ignition plug has been conducted, since the plasma jet ignition plug provides quick propagation of combustion and exhibits such high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
The plasma jet ignition plug has a structure in which an insulator formed from ceramic or the like surrounds a spark discharge gap between a center electrode and a ground electrode, thereby forming a small-volume discharge space called a cavity. An example system of ignition of the plasma jet ignition plug is described. For ignition of an air-fuel mixture, first, high voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge. By virtue of associated occurrence of dielectric breakdown, current can be applied between the center electrode and the ground electrode with a relatively low voltage. Thus, through transition of a discharge state from the spark discharge effected by further supply of energy, plasma is generated within the cavity. The generated plasma is jetted out through an opening (so-called orifice), thereby igniting the air-fuel mixture (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-294257, “Patent Document 1”).
Meanwhile, an example of the geometric shape of plasma jetted out from the opening is the form of a pillar of fire (hereinafter, such a form of plasma is referred to as “flame-like”). Because of extension in the direction of jet, the flame-like plasma is characterized by a large contact area with an air-fuel mixture and by high ignition performance.